The obstetrical applications of ultrasound have expanded during the last four decades , as sophisticated technologies have evolved. Although considered an essential method for assessing fetal status, there remain many gaps in our understanding of the interaction of this imaging modality with fetal tissues. Prior investigations with the monkey have shown that reduced birth weights and transient neutropenias result from frequent prenatal exposure. To further explore these findings, 26 animals were studied (16 exposed, 10 controls; "triple mode"; atl ultramark 9 with hdi~; isptad~645-714 mw/cm2). Exposures were performed every 5 days from gestational day (gd) 21-36 (5 min), every other day from gd 37-60 (5 min), then weekly from gd 61-153q1 (10 min). Fetal blood samples were collected for complete blood counts (CBCS), progenitor assay, circulating insulin-like growth factors (IGF-I, IGF-II) and binding proteins (IGFBP-3). Animals were delivered by cesarean section at term, and body weights, morphometrics, cbcs, and bone marrow aspirates assessed at delivery and postnatally (3 months). Fetal neutropenias were observed in exposed animals in addition to reductions in circulating progenitors (colony forming unit-granulocyte-macrophage [CFU-GML). Growth of CFU-GM from bone marrow was exuberant at term, whereas circulating levels were diminished compared to prenatal samples. Exposed animals were smaller at birth; marked reductions in fetal IGFBP-3 were noted. These data suggest that (1) frequent prenatal ultrasound exposure can transiently alter the neutrophil lineage which may be due to enhanced margination and/or organ sequestration, and (2) transient, altered growth patterns may be due to perturbations of the IGF axis. Continued studies with the monkey model will help further our understanding of potential interactions of this imaging modality with developing tissues, and help establish the mechanisms by which biologic effects may occur in developing mammalian systems.